If a person chooses to walk rather than drive, there is one less vehic

If a person chooses to walk rather than drive, there is one less vehicle emitting pollution into the air than there would be otherwise. Therefore if people would walk whenever it is feasible for them to do so, then pollution will be greatly reduced.

Which one of the following is an assumption on which the argument depends?

(A) Cutting down on pollution can be achieved in a variety of ways.

(B) Taking public transportation rather than driving is not always feasible.

(C) Walking is the only feasible alternative to driving that results in a reduction in pollution.

(D) There are people who never drive but who often walk.

(E) People sometimes drive when it is feasible to walk instead.
C and E are the two contenders.
C is good but if one can question that is walking the only alternative then one can be sure to eliminate it. It is about necessary or sufficient. Is it necessary that walking has to be the only alternative. No, not likely. There can still be others. Also, conclusion has a hint about how pollution reduction can be achieved. If we focus on how then we can certainly see why E is good enough a choice.

Answer E.

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If a person chooses to walk rather than drive, there is one less vehicle emitting pollution into the air than there would be otherwise.
Conclusion: If people would walk whenever it is feasible for them to do so, then pollution will be greatly reduced.

The conclusion is that if people walk instead of driving (whenever possible), then pollution will be greatly reduced.

What is the assumption here? What is it that the author has taken for granted? That currently there are cases when people choose to drive even though it is feasible to walk. Only then will choosing to walk if feasible lead to reduction in pollution.
If of every 100 drives, 80 are necessary but 20 could be converted to walk, then choosing to walk will reduce pollution.
But if all 100 drives are necessary and cannot be walked, then choosing to walk when feasible will not reduce pollution. There won't be any drives that will be converted to walks because it is not feasible to convert them to walks (too long or no walkway etc).

Hence answer will be (E)
(E) People sometimes drive when it is feasible to walk instead.

Look at the other options now.

(A) Cutting down on pollution can be achieved in a variety of ways.

Irrelevant. We are discussing this way of converting drives to walks only.

(B) Taking public transportation rather than driving is not always feasible.

We are not discussing taking public transportation. We are discussing walking.

(C) Walking is the only feasible alternative to driving that results in a reduction in pollution.

It is not necessary that walking is the "only" alternative to driving to reduce pollution. Pollution could be reduced by many other means too, say by taking public transport. But that doesn't impact our conclusion. We are talking about how walking instead of driving can reduce pollution. Just because another alternative can also reduce pollution, it doesn't impact our this argument. For our this plan to work, we don't need that walking should be the ONLY feasible alternative to driving. We just need it to be a feasible alternative to driving.

(D) There are people who never drive but who often walk.

These people are out of scope. They are anyway not causing any pollution. We are talking about those who are driving and hence causing pollution.

Answer (E)

Discussion on Assumption Questions:
https://youtu.be/O0ROJfljRLU